In this study, metal (Cu or Ni) seed is formed on the surface of PET film using atmospheric pressure plasma, and Ag layer is formed on the surface of metal seed / PET film. The morphology of the Ag layer and the electromagnetic interference (EMI) shielding effectiveness (SE) are investigated according to the presence of metal seed and the kind of metal seed precursor. The surface of the PET film is pretreated with an atmospheric plasma, and then a 10 mM metal precursor solution is loaded, dispersed and dried. Thereafter, the metal precursor dried on the surface of the PET film is reduced by atmospheric plasma. As a result, the Ag layers formed on the Cu seed/PET film and the Ni seed/PET film are more uniform and smooth than the Ag layer formed on the surface of the PET film. The former EMI SEs are higher than the latter one. In particular, the highest EMI SE is measured from the Ag layer on a Ni seed/PET film. The results of the present study are expected to lead to the efficient fabrication of metal or alloy layers on the surfaces of polymeric materials with low melting points at a relatively low cost for EMI shielding applications.